The present invention relates to a rotary electrical machine of the kind in which a plurality of permanent magnets are arranged around a rotor and a stator is provided with appropriate electrical windings. Such machines can act as electrical motors, i.e. produce rotary motion upon application of electricity to the stator windings. Alternatively, they can perform as electrical generators, i.e. alternators or dynamos, wherein rotary motion imparted to the rotor can produce an electrical output from the stator windings.
Machines of the aforementioned kind can be embodied in relatively compact efficient units. One application where small size, and high efficiency is called for is in automotive generators, for example automobile alternators. Many different alternator designs have been proposed since the inception of the internal combustion engine. One such proposal is disclosed in UK patent specification number GB-A-2 174 252.
However, the constant demand for reduction in manufacturing costs, less consumption of raw materials, lightweight components, etc., means that there is a need for even smaller and more efficient lightweight alternators.
A number of arrangements have been proposed for generating the field using permanent magnets instead of rotor field windings. This is particularly attractive now that high flux rare earth Neodymium Boron Iron magnets are available at a relatively low cost. One such arrangement is described in International patent application number PCT/GB96/01293, which relates to a rotor for an electrical machine, the rotor comprising a plurality of equi-angularly spaced magnets. The elimination of the field windings reduces the complexity of the machine in removing the need for slip rings, thereby also increasing the reliability of the machine. Since no field current is required, losses are reduced and efficiency is increased to up to twice that of a conventional claw pole device under typical operating conditions. Furthermore, in general, the power-to-weight ratio of a permanent magnet excited machine is much greater than that of the conventional claw pole machine, thereby enabling a much smaller, lower weight machine to be used for similar applications.
However, some difficulties have been encountered in the regulation of the voltage output of permanent magnet machines. Since the voltage is proportional to the speed of rotation of the rotor, the speed ratio often being as high as 10:1, any regulation means must be able to cope with a high voltage as well as having to regulate the full output current. The output voltage must be over the minimum supply voltage at the lowest speed, so that the voltage at the highest speed will be at least ten times this value, which can also have an adverse effect on the safety of the machine. In the arrangement described in International patent application number PCT/GB96/01293, the voltage is regulated by electronic means.
In the case of a conventional claw pole machine, voltage regulation is achieved by varying the field current (flowing in the rotor field windings) which causes a corresponding variation in the field strength. This has two main benefits. Firstly, since the field current is a fraction of the full output current, a relatively low cost regulator can be used.
Another problem associated with permanent magnet machines which may arise is the generation of excessive eddy currents, which in turn could cause unacceptably high levels of heat loss in the machine. The level of eddy currents is approximately proportional to the square of speed and the square of flux density. For a permanent magnet machine where the flux is at a constant value, the eddy current losses increase with the square of speed, hence finely stranded conductors are necessary to avoid excessive losses at high speeds. The use of finely stranded stator conductors substantially reduces the conductor copper density and, therefore, the power output at a given speed. Thus, because the copper packing density of the stator is reduced, a larger less efficient machine is required for a given power output. Furthermore, the use of finely stranded stator conductors results in poor heat transfer in the stator, such that the stator temperature is relatively high resulting in an unacceptably low power output, especially at high temperatures.
However, in the case of the claw pole machine, eddy currents are not substantially increased by an increase in speed because the flux density in a claw pole machine falls in response to an increase in speed.
In summary, therefore, although conventional permanent magnet machines are significantly more efficient and of lower weight than the claw pole machine, the difficulty in regulation often outweighs the benefits gained, particularly in high speed applications. This difficulty is quantified in terms of the cost of the regulation electronics and the weight and size of the machine.
It is an object of the present invention to provide an electrical machine having a rotor and a stator, the rotor comprising at least one magnet which is located adjacent to the stator with an air gap therebetween, the machine comprising means for varying the size of said air gap in response to a variation in the speed of rotation of said rotor.
At the lowest speed of rotation of the rotor, the air gap is of minimum size and the magnet is located as close as possible to the stator. Thus, the flux density is at a maximum level and a maximum voltage is generated. As the speed of rotation increases, the voltage generated should increase. However, as the speed of rotation increases, the magnet is moved away from the stator, thereby increasing the size of the air gap and decreasing the flux density at that speed. Thus, the otherwise increased voltage is compensated by increasing the size of the air gap, and the output voltage remains substantially constant. Thus, the need for complex and expensive circuitry to regulate the output voltage is substantially eliminated. Even if additional circuitry is required to assist in the voltage regulation, such regulating electronics would be relatively simple and less expensive than the circuitry required for voltage regulation in conventional permanent magnet machines.
Furthermore, as stated above, the level of eddy currents generated in the stator is proportional to the square of the frequency and the square of the flux density. As the speed of rotation increases, obviously the frequency increases accordingly. However, the flux density is reduced by the increase in size of the air gap, thereby compensating for the increase in flux density. In this way, any increase in eddy currents with the increased speed of rotation is substantially eliminated. In fact, because of this it is not necessary to use finely stranded conductors in the stator. Instead, the stator can be formed of copper sheeting, onto which the wiring pattern is etched, stamped out, or cut by other means, e.g. laser or water jet. As a result, the heat transfer characteristics of the stator are good because the copper packing density of the stator is high. Thus, the size of the machine required to give a particular power output can be reduced, and the cost of manufacture is also reduced. At higher speeds, high eddy current losses do not occur since the drop in field strength due to the increased air gap would compensate for the increase in frequency.
The size of the stator, and therefore the overall machine, can be further reduced by the addition of ferromagnetic material typically laminated soft iron therein, to assist in drawing more of the flux into the stator coils, thereby increasing the voltage generated. Furthermore, as the airgap increases, the flux going through the stator can be reduced by increasing the flux leakage. One or more rings of additional stationary iron of a particular shape may be placed in such a way as to assist the leakage of flux going away the stator coils. This has two main benefits: (i) the maximum movement of the rotor(s) is reduced; and (ii) the design the means for varying the airgap is easier, since the iron ring shape can be tuned to give the required voltage regulation.
The use of iron in the stator used in the present invention is permissible, because iron losses are not significantly increased at high speeds due to the resultant drop in field strength.
Thus, the machine of the present invention may comprise one or more stationary metal rings, preferably formed of iron, which is/are mounted concentrically with the stator with substantially no space therebetween. Alternatively, a stationary iron ring may be formed integrally with the stator. In a preferred embodiment, as the at least one magnet is drawn away from the stator and the airgap is increased, the at least one magnet is drawn closer to the stationary iron ring which assists in drawing away flux, thereby increasing the effectiveness of the regulation process. Furthermore, since the maximum airgap does not have to be so large, the compactness of the machine is also improved. The diameter of the iron ring(s) may be less than that of the stator. However, in a preferred embodiment the diameter thereof is greater than that of the stator. Soft iron inserts may also be placed in the stator to increase the voltage generated in the stator coils.
The means for varying the airgap preferably comprises one or more resiliently flexible members formed integrally with or mounted on the rotor, the resiliently flexible member co-operating with a respective magnet of the rotor so that, as the speed of rotation of the rotor increases the centrifugal force generated thereby causes movement of the resiliently flexible member or members and thereby draws the or each respective magnet away from the stator to increase the airgap. In a preferred embodiment, the rotor comprises a plurality of equi-angularly spaced magnets.
The means for varying the airgap is preferably mounted or biased such that when the rotor is stationary or at its lowest operating speed, the airgap is at a minimum. In one embodiment, the means for varying the airgap may comprise one or more members supported on a rotor drive shaft, the member or members being angled to be progressively closer to the stator with increasing radial distance from the drive shaft. In this case, an increase in rotor speed results in a centrifugal force to draw the angled portion of the airgap varying means back to a substantially vertical position at maximum rotor speed. For the avoidance of doubt, the term xe2x80x9cprogressivelyxe2x80x9d is not intended to mean only linear. The angled portion of the airgap varying means may, for example, be curved or stepped.
The machine may further comprise electronic means for additional voltage regulation. Because a large proportion of the voltage regulation is achieved by mechanical means, the electronic circuitry required for any additional voltage regulation is substantially simplified. Additional mechanical means, for example flyweights, may also be provided for assisting in drawing the magnets away from the stator to increase the airgap.
When current is drawn from the machine by a load, there is a corresponding drop in output voltage for a particular rotor speed. As the rotor speed itself is unchanged, such a voltage drop is not compensated for by the airgap varying means. Thus, in a preferred embodiment of the present invention, feedback means are provided for feeding back a portion of the output current to, for example, a solenoid which operates to push the at least one magnet back towards the stator and decrease the airgap by an amount corresponding to the drop in output voltage at that rotor speed.
According to another aspect of the present invention there is provided a stator for an electrical machine, the stator comprising electrical windings arranged as coil sectors disposed substantially equi-angularly in a generally circular pattern on two opposing sides of the stator, wherein at least some of the coil sectors are wound in a generally spiral fashion when viewed in the direction of the axis of symmetry of said generally circular pattern, characterised in that at least two of the coil sectors, one on each of the two opposing sides of the stator, are formed of a continuous electrical winding which passes through the stator from one side to the other side.
Alternatively, regulation which is necessary in order to compensate for changes in voltage according to changes in load may be carried out entirely by electronic means, such as, for example, a DC to DC converter placed after the voltage rectifier or a controlled rectifier.